1. Field of the Invention
The present invention generally relates to refrigerating machines and, more particularly, to a pulse-tube refrigerating machine.
2. Description of the Related Art
Conventionally, a pulse-tube refrigerating machine is used to cool an apparatus requiring an extremely low-temperature environment, such as, for example, a nuclear magnetic resonance apparatus (MRI).
FIG. 1 shows an outline structure diagram of a two-stage pulse-tube refrigerating machine as an example of a pulse-tube refrigerating machine. The two-stage pulse-tube refrigerating machine 10 comprises a housing part 1 and a cylinder part 2 connected to the housing 1.
The housing part 1 includes a housing 5 that accommodates therein component parts such as a reservoir, an orifice, a valve, piping, etc. A refrigerant gas such as helium gas or the like is supplied from a compressor 3 into the housing part under a high pressure at a predetermined cycle. The refrigerant gas is discharged from the housing part 1 under a low-pressure.
The cylinder part 2 is connected with the housing part 1 through a flange 11. The cylinder part 2 includes a first-stage pulse-tube 12 and a second-stage pulse-tube 13. The pulse-tubes 12 and 13 are fixed at their upper ends (high-temperature ends) to the flange 11. Further, the cylinder part 2 has a first-stage regenerator tube 14 and a second-stage regenerator tube 16. The upper end of the second-stage regenerator tube 16 is connected to a first-stage cooling stage 15, which is connected to the lower ends of the first-stage pulse-tube 12 and the first-stage regenerator tube 14. Additionally, a second-stage cooling stage 17 is connected to the lower ends of the second-stage pulse-tube 13 and the second-stage regenerator tube 16. The upper end of the first-stage regenerator tube 14 is fixed to the flange 11 similar to the first-stage pulse-tube 12 and the second-stage pulse-tube 13. Additionally, the upper end (high-temperature end) and the lower end (low-temperature end) of the first-stage pulse-tube 12 are connected with rectifying members 12A and 12B, respectively. Also, the upper end (high-temperature end) and the lower end (low-temperature end) of the second-stage pulse-tube 13 are connected with rectifying members 13A and 13B, respectively.
If the overall length of the above-mentioned pulse-tube refrigerating machine is long, a handling operation, such as installing the refrigerating machine to an apparatus to be cooled or removing the refrigerating machine from the apparatus to be cooled, becomes difficult. Thus, it is suggested to shorten the overall length of a pulse-tube refrigerating machine by substituting a portion of the upper end (high-temperature end) of the first-stage pulse-tube 12 or the second-stage pulse-tube 13 by a space formed in the housing 5 (refer to Patent Document 1).    Patent Document 1: Japanese Laid-Open Patent Application No. 2005-156029
However, if a space for the pulse-tube is formed in the housing 5, the refrigerant gas flowing through the pulse-tube is brought into contact with a wall of the housing forming the space. The housing part 1 made of aluminum or an aluminum alloy is at a substantially room temperature, while the refrigerant gas is at a relatively low-temperature. Accordingly, a heat transfer tends to occur in the housing wall, thereby generating a heat loss in the refrigerant gas.
Moreover, under such a contact state of the refrigerant gas and the housing wall, the characteristic of the refrigerant gas tends to be influenced by a change in the ambient environmental condition (for example, temperature change). This means that, if the settings for operating the pulse-tube refrigerating machine, such as, for example, a setting of open and close timing of an orifice, is optimized while expecting a certain characteristic of the refrigerant gas, an actual operation of the pulse-tube refrigerating machine is deviated from an optimum state due to a change in an ambient environmental condition.
Therefore, when the space for the pulse-tube is formed in the housing, there is a problem in that a finally-obtained refrigerating capacity of the pulse-tube refrigerating machine is reduced.
Moreover, even in a more conventional pulse-tube refrigerating machine (that is, the structure shown in FIG. 1) having no space for a pulse-tube in a housing, if the thickness of the flange is increased, a heat loss according to a heat transfer through a contact portion of the flange and the pulse-tube becomes not negligible. Therefore, the above-mentioned problem of the reduction in the refrigerating capacity also occurs in such a pulse-tube refrigerating machine.